How do I relax enough so my bf and I can have sex for the first time? I tense up my muscles when he starts trying to get in so how do I just relax?

Foreplay. This is not uncommon.Although it seems weird and uncomfortable, a discussion before you even begin can help.An assurance from him that he won't hurt you helps you relax.
Patience. Time, patience and reassurance on the part of your boyfriend will help. The basic question is what causes you to tense up. Are you having second thoughts about the sex, or are you apprehensive about the pain, afraid of std or pregnancy, or other issues. Talk this through with bf and proceed slowly, perhaps using digital expansion of the vaginal inlet before inserting penis.